Monday, July 18, 2011

Recently, I've been having a lot of fun learning about HBase and Hadoop. One esoteric thing I just learned about is the way that HBase tables are populated.

By default, HBase / Map Reduce jobs can only write to a single table because you set the output handler at the job level with the job.setOutputFormatClass(). However, if you are creating an HBase table, chances are that you are going to want to build an index related to that table so that you can do fast queries on the master table. The most optimal way to do this is to write the data to both tables at the same time when you are importing the data. The alternative is to write another M/R job to do this after the fact, but that means reading all of the data twice, which is a lot of extra load on the system for no real benefit. Thus, in order to write to both tables at the same time, in the same M/R job, you need to take advantage of the MultiTableOutputFormat class to achieve this result. The key here is that when you write to the context, you specify the name of the table you are writing to. This is some basic example code (with a lot of the meat removed) which demonstrates this.

Sorry lise, it looks like the api may have changed and I haven't kept up with Hadoop enough to know the right solution for you. I think, based on reading the javadoc, somehow, you want to get the writer, which is passed into the Context constructor and then write to that.